The dynamic nature of aTiO2: Implications for Ti-based thermometers in magmatic systems

In recent decades, new Ti-based thermometers have found widespread use in geosciences, providing a convenient and powerful tool for investigating the crystallization temperatures of quartz and zircons in magmatic systems. However, a commonly overlooked aspect is the constraint of TiO₂ activity (aTiO₂^{liquid-rutile}). Many studies assume aTiO₂ to be constant or equate the presence of Ti-rich phases, such as ilmenite, with fixed activity levels. Using solubility models and data from natural systems, we demonstrate that aTiO₂ is a dynamic parameter, influenced by temperature, mineral assemblage, and TiO₂ content in the melt. Focusing on examples from several volcanic fields (Bishop Tuff, Fish Canyon Tuff, Yellowstone, and Shiveluch), we discuss the impact of these factors on aTiO₂ and highlight how inadequate constraint of aTiO₂ can lead to erroneous interpretations of magma storage conditions.